The present invention generally relates to communication systems, and in particular to a system and method for providing a communication system that operates in multiple communication modes.
Radio frequency devices communicate over different frequency ranges when operating in different communication standards or modes. For example, radio frequency devices employing the Global System for Mobile Communications (GSM) standard operate within a relatively high frequency range, while some other radio frequency devices utilizing other communication standards operate within lower frequency ranges, such as the Advanced Mobile Phone Service (AMPS) and the Integrated Digital Enhancement Network (iDEN). Other standards such as Personal Communication Service (PCS), United States Digital Cellular (USDC), Code Division Multiple Access (CDMA) also provide for radio devices to operate in different frequency ranges.
Phase locked loop (PLLs) devices and voltage controlled oscillators (VCOs) have been used in conventional radio systems to supply a local oscillator (LO) signal for a radio""s receiver. The incoming RF signal is mixed with the LO signal to provide a lower intermediate frequency at which filtering and amplification are more readily achieved. Achieving the filter selectivity needed to reject interfering signals during the amplification and filtering of the incoming RF signal is simpler and more economical at a lower intermediate frequency, which is well known to those skilled in the art of radio system design.
A radio receiver may be a double super heterodyne receiver, which employs a first and a second LO signal. The first LO produces a first local signal which is related to the reference signal as regards to its frequencies. The second LO produces a second local signal which is not related to the reference signal in frequency. A mixing section is provided that includes a first and second mixing unit. The first mixing unit may be connected to the receiving section and the first LO for mixing the received signal with the first local signal to produce a first intermediate (IF) signal. The second mixing unit is connected to the first mixing unit and the second LO for mixing the first IF signal with the second local signal to produce a second IF signal.
Some radio frequency devices are designed to employ multiple modes or standards. These devices allow for selection between the different modes or standards by a switch, a programmable input or some other mechanism. Multiple mode radio systems employ separate dedicated PLLs and VCOs to provide the second LOs per mode of operation for multimode receivers used in multimode radio systems. A single VCO and a PLL may include over 25 parts and takes up a substantial amount of cost and real estate with regards to the entire radio device. Therefore, it is desirable to provide a less expensive and more convenient approach for providing a second LO signal to a second mixer to generate a second IF signal for a receiver for each mode in a multimode radio system.